The invention relates to metallurgy. More precisely, it relates to carbon steels of the type which are to undergo galvanization, that is to say the deposition of zinc on their surface by immersion of the product in a bath of liquid zinc. The product is then generally in the form of a running strip or of a sheet.
Carbon steels for galvanization are steels comprising not more than 0.15% carbon and from 0.08 to 2% manganese, as well as the alloying elements and impurities conventional in carbon steels. The various classes of steel for galvanization are distinguished essentially by their contents of deoxidizing elements.
So-called “class 3” steels have a silicon content of from 0.15 to 0.25%.
So-called “class 2” steels have a silicon content less than or equal to 0.040%.
So-called “class 1” steels have a silicon content less than or equal to 0.030%.
The production and continuous casting of class 3 steels do not give rise to particular problems because, as a result of their silicon content, that element controls the deoxidation of the liquid steel by forming oxidized inclusions with the dissolved oxygen (optionally in combination with manganese).
For that reason, CO formation within the liquid steel, which would be likely to cause rimming of the steel at the time of casting, is not observed.
The same does not apply in respect of class 1 and 2 steels. In their case, the silicon content is too low for that element to intervene in the deoxidation process. It is the carbon that controls the deoxidation, and this manifests itself in the formation and evolution of CO, rendering the steel “rimmed”. This rimming has two disadvantages:                on the one hand, during solidification of the steel, it often causes the appearance of “blowholes” in the central region of the product, that is to say pores corresponding to the location of pockets of gas present at the moment of solidification; however, this disadvantage can be overcome if the steel subsequently undergoes vigorous hot rolling, which will close the pores;        on the other hand, if the rimming unexpectedly becomes too great, there is a risk of the steel overflowing from the ingot mold in which solidification is taking place.        
This latter risk is especially to be feared when a steel is cast continuously on a machine of the conventional type having a cooled and oscillating bottomless ingot mold with fixed walls. If the steel present in the ingot mold overflows, it represents a danger to surrounding personnel and leads to serious damage to the casting machine.
For that reason, sheets and strips of class 1 and 2 steel are conventionally obtained from intermediate products which are:                either cast not continuously but in ingots in a conventional ingot mold, because this process is more tolerant of possible rimming of the steel: filling of the ingot mold can be discontinued before it overflows if pronounced rimming is noted, and even the consequences of an overflow are never serious to the point of calling into question the smooth running of the steel works; the ingots are subsequently hot rolled to form slabs;        or cast continuously in the form of slabs on conventional machines having a cooled oscillating bottomless ingot mold with fixed walls, but after addition to the steel of a relatively large amount of aluminum so that that element controls the deoxidation by forming solid alumina inclusions, thus preventing the formation of CO and, accordingly, rimming.        
These two methods are not ideal, however. It is well known that casting in ingots is less productive than continuous casting and subsequently requires a larger number of hot rolling steps to obtain a product of a given thickness. With regard to deoxidation with aluminum, it is more costly in terms of alloying elements. In addition, the alumina inclusions must be removed as far as possible prior to the continuous casting step so that there is no risk of their blocking the nozzles of the distributor of the casting machine.
Aluminum inclusions can be made liquid by treatment with calcium, but this introduces an additional cost in terms of alloying elements. It is also necessary to prevent as far as possible atmospheric reoxidation during the continuous casting, in order to avoid the formation of new alumina inclusions which it will not be possible to remove before solidification and which will be found in the end product, whose mechanical properties they will impair. To that end, argon is injected into the nozzles introducing the steel into the ingot mold, which, again, increases the manufacturing cost. In addition, there is a risk of bubbles of argon becoming trapped at the time of solidification, which are liable to cause faults in the product.
It would, however, be valuable to manufacture class 1 and 2 steels for galvanization by a process that is as economical as possible, because such steels have the advantage of allowing higher rates of deposition of the galvanizing coating than class 3 steels. This advantage is scarcely noticeable when the galvanization is effected by unrolling a strip of steel in a bath of liquid zinc. On the other hand, when an isolated sheet is immersed in the bath of zinc, it is important for the quality of the product and the productivity of the installation that the deposition be as rapid as possible.